A. Technical Field
The present invention relates generally to a system and method for sensing and analyzing certain gases, including NOx, hydrocarbons, carbon monoxide and oxygen in a multi-component gas system using chemical sensors and chemical sensor arrays.
B. Description of the Related Art
The use of chemical sensing devices to detect certain gases is known. Many attempts have been made to find a material with selectivity and sensitivity for a specific gas. For example, U.S. Pat. No. 4,535,316 discloses a resistive sensor for measuring oxygen. See also H. Meixner et al., Sensors and Actuators B: Chem., vol. 33, pp. 198-202 (1996). It is apparent that different materials must be used for each gas to be detected. However, when a gas is part of a multi-component system, using one material to detect a specific gas is difficult because of the cross-sensitivities of the material to the various component gases of the mixture.
One example of a multi-component gaseous system is a combustion gas emission, which can include oxygen, carbon monoxide, nitrogen oxides, hydrocarbons, CO2, H2S, sulfur dioxide, hydrogen, water vapor, halogens and ammonia. See H. Meixner et al., Fresenius' J. Anal. Chem., vol. 348, pp. 536-541 (1994). In many combustion processes, there is a need to determine whether the gas emissions meet requirements established by federal and state air quality regulations in various jurisdictions. Several types of gas sensors have been developed to address this need. See U.S. Pat. No. 5,630,920, which discloses an electrochemical oxygen sensor; U.S. Pat. No. 4,770,760, which discloses a sensor for detecting oxygen and oxides of nitrogen; and U.S. Pat. No. 4,535,316, which discloses a resistive sensor for measuring oxygen. It would be advantageous to be able to simultaneously analyze two or more components of a mixture such as a combustion gas emission, to calculate concentration for example, in terms only of data generated by direct contact of the gases with a sensor and without having to separate any of the gases in the mixture. Prior art methods do not currently meet this need.
Numerous sensors have been disclosed to detect gases evolving from foods and from other relatively low temperature applications. See K. Albert et al., Chem. Rev., vol. 200, pp. 2595-2626 (2000). Arrays of several undoped and doped tin oxide sensors have also been disclosed for use in detecting various combustion gases up to 450° C. See C. DiNatale et al., Sensors and Actuators B: Chem., vol. 20, pp. 217-224 (1994); J. Getino et al., Sensors and Actuators B: Chem., vol. 33, pp. 128-133 (1996); and C. DiNatale et al., Sensors and Actuators B: Chem., vol. 23, pp. 187-191 (1995). However, at higher temperatures and in the highly corrosive environment in which one would use chemical sensors to monitor combustion gases, operating temperature can alter or impair the performance of the sensor array. That being the case, high temperature environments require the use of materials that are both chemically and thermally stable and that maintain measurable responses to the gases of interest. The effect of the operating temperature on the response of tin oxide-based sensor arrays was studied up to 450° C. See C. DiNatale et al., Sensors and Actuators B: Chem., vol. 23, pp. 187-191 (1995). However, materials in addition to those previously known in the art are still needed to be able to provide a system and method capable of directly monitoring the gas emissions of multi-component gas systems at higher temperatures, such as would be encountered in the operation of combustion gas systems. Detecting circuits and analytical devices are also needed for such chemical sensors to provide a system and method capable of processing the chemical sensor outputs and providing useful information in determining the components and constituents of the monitored multi-component gas system.
Addressing this need would permit the use of a chemical sensor to measure combustion emissions, such as automobile exhausts, and determine whether those emissions meet functional and mandated requirements. In addition, it has surprisingly been found that the system and method of this invention that are useful for analyzing high temperature gases, such as automotive emissions, may be employed with equal effect in analyzing low temperature gases.